This proposed research is concerned with the molecular mechanisms involved in the control of virus gene expression. We proceed with the bacteriophage T7- Escherichia coli system as a simple virus-host model system, and aim to obtain thorough understanding of virus development in the host which is applicable for the studies of other viruses including animal viruses and tumor viruses. 1. The "early to late" switch in T7 phage development depends on a transcriptional switch from the use of the host E. coli RNA polymerase to T7-coded RNA polymerase. In addition, T7 phage has a "host shut-off" function which results in an inactivation of the host RNA polymerase thus terminating the synthesis of host RNA and T7 early mRNA. An inhibitor protein of E. coli RNA polymerase has been isolated and purified from T7 infected cells. We found that T7 gene 2 codes for this inhibitor protein, termed I protein (Mr=7000-9000). Since T7 gene 2 is essential for the synthesis of T7 progeny DNA, probably due to its involvement in the host restriction system, we are investigating this dual function of gene 2 product. 2. New E. coli mutants have been isolated. T7 DNA processing and packaging are interfered in these mutants, resulting in an accumulation of T7 progeny DNA which is cleaved shorter than the T7 genome concomitant with an accumulation of large amounts of prohead and newly found head precursor particle, termed X particle. T7 DNA processing and head assembly process will be investigated using these host mutants.